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From Joshua Marantz <>
Subject Re: Vary:User-Agent, best practices, and making the web faster.
Date Sat, 04 Jun 2011 22:34:05 GMT
It was with some reluctance that I brought this up.  It occurs to me that
this idea propagates the sort of spec violations that led to this issue
(inappropriate user of Vary:User-Agent) in the first place.   However, I'm
trying to figure out how to improve compliance to support legitimate uses of
Vary:User-Agent without causing mod_pagespeed to become significantly less
ineffective across a broad range of sites.

We have found that putting complaints in Apache logs mostly causes disks to
fill and servers to crash -- although that does get it noticed :).  The
problem, put another way, is that mod_pagespeed cannot distinguish
legitimate uses of Vary:User-Agent, so it really has no business complaining
in logs.  Complaining in docs is fine; but some existing mod_pagespeed users
that simply type "sudo yum update" will later notice a performance-drop and
may not consult the docs to figure out why.

I'm also trying to grok the first response from Eric:

It's because of the other (dated) canned exceptions that set/unset
no-gzip/gzip-only-text/html based on the User-Agent, to second-guess
browsers that send AE:gzip but can't properly deal with it.

Going backwards:  which browsers send AE:gzip but can't properly deal with
it?   Does IE6 have that issue or is it only true of IE5?   I know that IE6
has had issues with compression in the past but they appear to be addressed
by patches issued by Microsoft four and a half years ago:;en-us;Q312496.  Moreover
IE6 is shrinking in market
10%) and IE5 does not appear in the pie-chart at all.

And I still don't understand how that relates to Vary:User-Agent.  What's
really at issue here seems more related to proxies; is that right?  That
proxies were not respecting Accept-Encoding, but sending gzipped content to
browsers that did not want it?  Is that still a problem?  Which proxies were
broken?  Are they still broken?

And, while I understand the reluctance to help me figure out from our module
what values were passed to SetEnvIfNoCase and Header, I would like to see
whether there's agreement that the Apache 2.2 docs for mod_deflate are no
longer appropriate -- and in fact harmful.


On Sat, Jun 4, 2011 at 5:03 PM, Ben Noordhuis <> wrote:

> On Sat, Jun 4, 2011 at 21:26, Joshua Marantz <> wrote:
> > I think what we'd do is basically let mod_pagespeed ignore
> "Vary:User-Agent"
> > if we saw that it was inserted per this exact pattern.  This would, to be
> This seems like a stupendously bad idea. Warn about it in your docs,
> complain about it in the logs but don't willy-nilly override people's
> settings.

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